For the entire Earth's climate history, it is not one core from one place that will tell us everything. It's why climate catastrophe denialists keep bringing up the "medieval warming period" which anybody in the field knows was a local anomaly, not a global one (or you'd see it reflected in records of other places). So standalone, the Greenland ice core is quite accurate for the local climate of Greenland. In context with the various measures of our past climate, it contributes another degree of certainty to the consensus global climate, more accurately reflecting the overall global climate.

Slight tangent but this phenomenon of local vs global is quite important when we think about other questions too, e.g. does the COVID vaccine cause heart disease (insert one statistic from one country that seems to correlate the two) - here you can ask if this is reflected in all the other countries as well, or might more related to something local (which could be as benign as how data was collected in one place)

I agree. It is however in different contexts an interesting question which evil would weigh heavier: The one intentionally inflicted with a reason behind it, or the one inflicted out of callous disregard.
IIRC Chomsky made an argument about that that bombing the Al-Shifa factory should be considered a worse evil than the twin tower attack in the sense that both were terrorist attacks (as long as you recognize state terrorism as terrorism) with similar order of magnitude of death toll - however intentional killing in a perverted sense at least recognizes the human status of the victim, while the Clinton Administration simply did not consider it important that thousands would die in some poor african nation somewhere as a consequence.